This invention relates to area navigation systems, and more particularly to an area navigation system capable of providing range and bearing data in conjunction with ILS and Localizer approaches. The invention is described in the particular context of the area navigation systems described and claimed in Assignee's Abnett et al U.S. Pat. No. 3,796,867 entitled, "ANGLE MODE AREA NAVIGATION COMPUTER", in Assignee's Bean U.S. Pat. No. 3,750,942 entitled, "DISTANCE MODE AREA NAVIGATION COMPUTER", and in Assignee's copending Foster et al U.S. Patent application Ser. No. 476,513, filed June 5, 1974 and entitled "AREA NAVIGATION SYSTEM." The disclosures of these patents and application are incorporated herein to extent pertinent.
An extended description of the concepts of area navigation and its many advantages is set forth in the Abnett et al and Bean patents, and the reader is referred to these patents for more information. Briefly, however, there exists a network of navigation aids called VORTAC's which provide radio information from which range and bearing information may be derived. In area navigation, the VORTAC range and bearing data are used in conjunction with arbitrarily selected range and bearing from the VORTAC to a so-called waypoint. Because the location of a waypoint is arbitrary within the range of a particular VORTAC, the pilot is no longer constrained to fly along or near the established airways (defined with respect to a series of VORTAC stations).
In broad terms, area navigation systems such as those of the Bean and Abnett et al patents operate by solving a vector triangle defined by the aircraft, the VORTAC, and the waypoint, given the selected waypoint address vector and the measured aircraft to VORTAC vector.
In the Abnett et al patent, there is shown an area navigation system in which the aircraft to waypoint vector is used to generate a display of range from the aircraft to the waypoint, and a track angle deviation display. The latter is based on the difference between the actual aircraft to waypoint bearing (the angle component of the aircraft to waypoint vector) and the bearing at which it is desired that the aircraft intercept the waypoint, i.e. the selected track angle. The angle display is advantageously accomplished by means of a "left-right" needle included in the course deviation indicator (Or C D I) of the standard VORTAC navigation systems. In the Bean patents, track deviation is displayed on a left-right meter as distance from the selected track.
The RNAV equipment is, of course, not the only available navigation aid. In fact, instrumentation normally includes one, and usually two VOR receives and a DME (distance measuring equipment) receiver along with associated display equipment to process VORTAC data and for I L S (instrument landing system) approaches available at commercial airports and at many general aviation facilities. An RNAV system coupled with the above utilizes the same receivers and display equipment.
Moveover, there is a substantial degree of standardization. For example, the display equipment for the #1 or primary NAV receiver is on the left side and the display for the #2 NAV receiver is on the right. The left side display is used for I L S operation and for RNAV when present. Thus, in heretofore available installations during an instrument approach, RNAV data is not available.
This is undesirable for several reasons. For example, the front (normally used) course at an I L S equipped airport includes a pair of "Markers" giving distance fixes to an approaching aircraft. These are the LOM, or locator outer marker, normally located five miles from the end of the runway and the middle marker, located one-half mile from the runway. These markers actuate alarms in the aircraft to signal passage of the points in the approach. However, the I L S equipment makes no provision for determining distance to the marker. Such information could be extremely useful in assisting the pilot in adjusting the aircraft configuration (speed, flaps, etc.) and runway approach angle. Further, where ground control radar is available, one of its principal functions is to provide the pilot with distance to the L O M. Availability of RNAV during an ILS approach would be a most desirable substitute for ground control radar or for the so called terminal DME facility which are not commonly available at general aviation airports.